(1) Field of the Invention
The present invention relates to a innovative configuration of a low pressure chemical vapor deposition, (LPCVD), and gas feed system, used to grow materials, specifically polysilicon, used in the fabrication of semiconductor devices.
(2) Description of Prior Art
The semiconductor chip industry has been able to continually reduce cost of their products, while still providing increased performance and reliability. Most obvious to those skilled in the art has been the trend to sub-micrometer images, allowing more circuits to fabricated per chip, resulting in large increases in density with corresponding reductions in cost. This has been accomplished primarily by the advances in photolithographic technology. Greater sophistication in cameras, as well as the use of more sensitive photoresist materials, have allowed the trend of microminiturazation to advance at a rapid pace. Other disciplines, also needed in the semiconductor fabrication sequence, such as reactive ion etching, (RIE), and ion implantation, (I/I), have also experienced similar optimizations. The use of RIE processes have allowed the sub-micrometer images, created via use of the advanced photo cameras and sensitive photoresists, to be easily transferred to the underlying material. The ability of RIE to anisotropically remove semiconductor materials, without eroding the overlying photoresist mask, and being able to stop, or slow down the removal process at endpoint, has also contributed to the growth of microminituraztion. Ion implantation has allowed for precise levels of impurities, needed to obtain the desired electrical result, to be placed in specific areas of the semiconductor chip. Thus it can seen that contributions to the development of smaller, faster, less costly semiconductor chips, have come from several chip fabrication areas.
One discipline, also essential for the fabrication of semiconductor chips, is chemical vapor deposition, (CVD). The ability to deposit various materials, that are used for semiconductors, that would otherwise be difficult to achieve via other semiconductor processes, has also been a major factor in growth of this industry. Materials such as silicon nitride, low temperature silicon dioxide, etc, have been routinely obtained using CVD processing. The development of low pressure chemical vapor deposition, (LPCVD), has rewarded users with a more advanced deposition product. The LPCVD process results in improvements in the thickness and resistivity uniformity, regards to within a wafer and also from wafer to wafer. This has allowed depositions to be performed on large quantities of wafers, in a single sequence, resulting in excellent thickness uniformity. However one type of LPCVD process, the deposition of amorphous or polycrystalline silicon, still has not reached the level of thickness and resistivity uniformity desired by semiconductor engineers. "The LPCVD Polsilicon Phosphorous Doped in Situ as an Industrial Process", by A. Baudrant, et al, in J. Electrochem. Soc. Solid State Sci. and Tech., May 1988, pp. 1109-1115, describes the use of a specific set of reactant concentrations that result in an improvement in thickness and resistivity uniformity improvement. Another article, "Kinetics and Mechanism of Amorphous Hydrogenated Silicon By Homogeneous Chemical Vapor Deposition", by B. A. Scott et al, in Applied Physics Letters, July 1981, shows that the critical event in achieving uniform polysilicon films, via CVD processing, is to develop a homogeneous deposition process. The routine method to grow LPCVD polysilicon is to inject a specific amount of silane, SiH4, into the reactor tube that contains the wafers waiting to be coated. Scott suggests that this results in a heterogeneous reaction., since the silane first, has to reach the temperature of the reactor tube, then decompose to silylene, (SiH2), and hydrogen, (H2), and finally deposit silicon on the wafer. The many stages result in poor uniformity.
This invention will desribe a novel apparatus that allows the silane gas to decompose to silylene and hydorgen, prior to reaching the reactor, thus a homogeneous process is achieved highlighted by significant improvements in both thickness and resistivity uniformity.